6,6′-Ditriphenylamine-2,2′-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties

A 2,2′-bipyridine with bulky triphenylamine substituents in the 6 and 6′ positions of the ligand (6,6′-ditriphenylamine-2,2′-bipyridine, 6,6′-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6′-diTPAbpy system to generate heteroleptic [Cu(6,6′-diTPAbpy)(bpy)]+ complexes were unsuccessful with only the [Cu(6,6′-diTPAbpy)₂](PF₆) complex observed. The 6,6′-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)₃Cl] and [Ru(CO)₂Cl₂] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6′-diTPAbpy)Re(CO)₃Cl] complex where the Re(I) ion is forced to sit 23° out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6′-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM^-1 cm^-1) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6′-diTPAbpy)₂]⁺ 353 nm, 72 mM^-1 cm^-1, [Ag(6,6′-diTPAbpy)²]+ 353 nm, 75 mM^-1 cm^-1), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the ¹ILCT state is observed at 436 nm (τ = 4.4 ns) for 6,6′-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional ³MLCT emission is observed for [Re(6,6′-diTPAbpy)(CO)₃Cl] (640 nm, τ = 13.8 ns). No emission was observed for [Ru(6,6′-diTPAbpy)(CO)₂Cl₂]. Transient absorption measurements revealed the population of a ³ILCT state for the Cu and Ag complexes (τ = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.